1. Field of Invention
This invention relates to golf clubheads with enhanced moments of inertia along both the vertical twist and the horizontal loft axes through the geometric center of the striking face to reduce twisting and loft changes, respectively, when a golf ball is struck.
2. Description of Prior Art
Heretofore designers have tended to approach the problem of the two-dimensional inertial stability of a golf clubhead by providing variations of the cavity-back design. The cavity, sometimes oval in shape, is positioned on the rear of the clubhead behind the striking face. For heel-shafted clubs, the walls of the cavities may not be uniform in thickness due to the twin requirements for a low center of gravity and for a toe section that is heavier than the heel section. Accordingly, the sole wall of the cavity may be thicker and heavier than the top wall, and the toe wall may be thicker and heavier than the heel wall.
The origin of the cavity-back design may be traced to U.S. Pat. No. 1,671,936, May 29, 1928. For driving and other distance irons. Sime taught decreasing the width of the blade in the middle and increasing the width along the heel and toe, respectively. It is seen that this configuration would tend to enhance the moment of inertia along the vertical twist axis and thereby reduce the twisting of the clubhead to help prevent a hook or a slice when a ball is struck.
On the other hand U.S. Pat. No. 2,254,528, Sept. 2, 1941 by Hoare taught decreasing the width of the blade in the middle and increasing the width along the top and bottom, respectively. It is seen that this configuration would tend to enhance the moment of inertia about the horizontal loft axis and thereby reduce any loft change of the clubhead to help prevent being long or short when a ball is struck.
A combination of these ideas into a cavity back iron to assist in the simultaneous prevention of hooking or slicing and falling long or short may be found in U.S. Pat. No. 2,846,228, Aug. 5, 1958 by Reach. In Reach's clubhead the cavity could be filled with a light synthetic rubber matrix which, in turn, could hold a variable amount of a material such as lead oxide to meet the individual requirements of a golfer for weight.
A more contemporary cavity-back design for a correlated set of iron clubs may be seen in U.S. Pat. No. 3,655,188, Apr. 11, 1972 by Solheim. Here. the individual requirement of a golfer for weight was met y controlling the depth of the cavities.
An extension of this design may be seen in U.S. Pat. No. 4,621,813, Nov. 11, 1986, also by Solheim. Here, each of the heads in a set of iron clubs contained a sole with the trailing edge indented inward and upward at the middle. Material was redistributed to the heel and toe to further enhance the moment of inertia along the vertical twist axis. The basic teaching of the cavity-back configuration for the set of clubs was left intact.